1. Field of the Invention
The present invention relates generally to equalization techniques for digital modulation such as QAM (quadrature amplitude modulation), and more specifically to a tap-weight controller of a matched filter receiver used in combination with an equalizer such as decision feedback equalizer.
2. Description of the Related Art
In digital radio transmission systems, intersymbol interference is equalized using a decision feedback equalizer when the ratio of undesired-to-desired signal (.rho.) is smaller than 1. However, if the desired, direct-path signal arrives at a receiver later than an undesired, indirect-path signal, interference occurs between them, causing what is called multipath fading. When this occurs, the amplitude of the undesired signal is stronger than the desired signal, hence .rho.&gt;1, giving rise to a large negative peak at time t=-T (where T is the symbol timing) with respect to the center, positive peak at the origin of the impulse response of the transmission system, whereas, when .rho.=0, i.e., in the absence of the undesired component, the impulse response is symmetrical with respect to the origin. Under such circumstances, intersymbol interference cannot completely be equalized.
It is known that the transfer function of an optimum filter is the complex conjugate of the spectrum of the input signal and such a filter is called a matched filter. Since the impulse response of the matched filter is a time-reversed and delayed version of the input signal, the application of a signal whose U/D ratio is smaller than 1 to the matched filter results in the generation of two negative peaks, one at time t=-T and the other at t=T, both having amplitudes one-half of the amplitude of the original negative peak at time t=-T.
A proposal has been made for using an adaptive matched filter in combination with a decision feedback equalizer in order to take advantage of the unique characteristic of the matched filter for equalizing digital signals affected during transmission by multipath fading since the two negative peaks at the output of the matched filter can be easily equalized by the decision feedback equalizer. The combined use of an adaptive matched filter and a decision feedback equalizer enables significant improvement of equalization when the later-arriving, indirect-path signal is dominant in the received signal. More specifically, the adaptive matched filter is implemented with an A/D converter for converting a demodulated incoming signal into digital samples a higher rate than the symbol rate of the incoming signal, a transversal filter and a tap-weight controller to control the tap weights of the transversal filter in an adaptive fashion in response to the output of the decision feedback equalizer. The tap-weight controller comprises a plurality of cross-correlators corresponding to the tap-weight multipliers of the transversal filter to detect cross-correlations between output digital samples of the equalizer derived from each incoming symbol and delayed versions of the corresponding digital samples supplied to the input of the adaptive matched filter. The correlation output signal from each cross-correlator has the same number of bits to represent a tap-weight coefficient as the number of quantization levels of the A/D converter. However, when the cross-correlation is low and hence satisfactory equalization cannot be expected, the lower bits of the tap-weight control signal are insignificant, and produce an undesired effect as if they were an additional source of intersymbol interference.